We all know how garden plants are protected from frost by a simple 

 covering of paper or clotli. Do we protect them in this way on a 

 windy or clondy night? The air under the protecting paper is not 

 notably warmer than the air at the same height outside. What is the 

 principle involved? In the early evening many objects, such as plants 

 and things made of metal, lose heat faster than they receive it. It 

 leaves in straight lines that point to the sky. These bodies lose heat 

 faster than the air does; they therefore become cooler than the air 

 above them. A frost may form on plants when the air four feet 

 above them is at 40 degrees. If some body be placed between them 

 and the clear sky the heat cannot readily pass off, and they remain at 

 the temperature of the air. This body may be a paper, a layer of 

 smoke or a cloudy sky. 



But what is the frost? The air always contains some water in the 

 form of invisible vapor. The amount is not the same at all times. 

 One cubic foot of the air contains an average of about 1.3 grains of 

 water in January and 6.5 grains in July. Heat perhaps has something 

 to do with the amount of water vapor in the air? Yes; the temper- 

 ature of the air determines the amount of water vapor it can contain. 

 At our average January temperature (26.6 degrees) a cubic foot of air 

 could not contain over 1.8 grains, and at our average July tempera- 

 ture (7-1.8 degrees) not over 9.3 grains of water vapor. 



What happens when warm, moist air cools? Our air usually con- 

 tains seven-tenths as much water vapor as it can hold. A cubic foot 

 of air at 50 degrees would then contain about three grains. This is 

 the total amount that air at 41 degrees can contain. If this air cools 

 to 38 degrees it cannot hold all the water vapor. If the air be in 

 contact with leaves or metals or other objects that lose their heat 

 readily, the ■^^•ater from the air will condense on them, and we call it 

 dew. If a cubic foot of the air at 50 degrees had contained but two 

 grains of water vapor it could have cooled to 30 degrees before any 

 of the \vater would have been deposited. But this is below the point 

 at which water freezes. The vapor would then have been deposited in 

 the form of minute ice crystals whicli we call frost. If a cubic foot of 

 the air at 50 degrees had contained but one grain of water vapor it 

 could have cooled to 11 degrees before any of the water would have 

 been deposited. But plants would have frozen before any such tem- 

 perature had been reached. Under these conditions the tem- 

 perature would not usually fall to 11 degrees, and no ice particles 

 would lie deposited on the outside of the plants. This is the dreaded 

 Mad: frost. 



